Positron Emission Tomography (hereinafter referred to as PET for short) is a non-invasive imaging method which can non-invasively, quantitatively and dynamically assess the metabolism, biochemical reactions, functional activities and perfusion of various organs of human body. Therefore, PET is used for early diagnosis and analysis of tumors, cardiac diseases and neurological diseases and plays a unique role in the prevention and treatment of serious diseases. During a PET imaging, it is needed to inject a drug marked with radioisotopes into a human body, an animal or an organism under detection. In the tissue of the object under detection, these radioisotopes encounter electrons and annihilate to generate a pair of γ photons. A detector at the periphery of the object under detection receives the γ photons and converts them into electrical signals. A series of processes are performed on these electrical signals, and an activity distribution of the object under detection is obtained by image reconstruction. [Miles N. Wernick, John N. Aarsvold, Emission Tomography: The Fundamentals of PET and SPECT, Elsevier Academic Press, 2004]
PET mainly includes a detector module, an electronics module and an image reconstruction module. The detector module receives and deposits γ photons and converts the γ photons into electrical signals; the electronics module processes and transmits these electrical signals the image reconstruction module processes the signal obtained by the system to obtain an image of activity distribution of the object under detection. After a PET system is installed, the detector module is fixed during a detection process or rotates around a fixed center in a fixed pattern [Michael E. Phelps, PET Physics, Instrumentation, and Scanners, Springer, 2006]. Moreover, for one object under detection, generally only one detection is performed or multiple independent detections are performed, and the layout and performance of the detector module are not adjusted in accordance with the characteristics of the specific object under detection.
Nowadays, animal PET achieves a better performance in spatial resolution, timing resolution, energy resolution, sensitivity, counting rate and so on as compared with PET for human body (hereinafter referred to as “human PET” for short, “PET” also refers to “human PET” unless otherwise indicated). The main reason lies in that, due to the partial volume effect, a better design scheme of the detector module is required for the purpose that the animal PET achieves the same performance as the human PET. If the design scheme of the detector module of the animal PET s used for the human PET, the ratio of the cost of the scintillation crystals between the human PET or the animal PET is proportional to the square of the ratio of the radius of a detection ring. Assuming that the field of view in the vertical axial direction (hereinafter referred to as FOV for short) of the human PET is 60 cm and for the animal PET it is 12 cm, and the axial FOV of the human PET and animal PET is same, the cost of the scintillation crystals for the human PET is at least 25 times as big as that of the animal PET.
The property of spatial resolution is taken as an example to illustrate the difference between the human PET and the animal PET. Spatial resolution is one of the most important performance indexes of PET. The higher spatial resolution means that it is able to detect a smaller lesion. Since the lesion size of an early cancer is commonly small, a PET with higher spatial resolution can improve the detecting rate for the early cancer. In the past, much work has been made to improve the spatial resolution of the PET system. The spatial resolution is mainly limited by the intrinsic spatial resolution of the detector, positron range, non-collinearity and so on [Craig S Levin, Edward J-Hoffman, “Calculation of positron range and its effect on the fundamental limit of positron emission tomography system spatial resolution,” Physics in Medicine and Biology, vol. 44, pp. 781-799, 1999]. Currently, for the human PET, the spatial resolution is about 2 mm˜10 mm Full Width at Half Maximum (hereinafter referred to as FWHM for short), the FOV in the vertical axial direction is approximately 50˜70 cm, the width of the scintillation crystal in the tangential direction is generally about 4 mm˜8 mm [F Lamare, A Turzo, Y Bizais, C Cheze Le, Rest, D Visvikis, “Validation of a Monte Carlo simulation of the Philips Allegro/GEMINI PET systems using GATE,” Physics in Medicine and Biology, vol. 51, pp. 943-962, 2006] [Brad J. Kemp, Chang Kim, John J. Williams, Alexander Ganin, Val J. Lowe, “NEMA NU 2-2001 performance measurements of an LYSO-based PET/CT system in 2D and 3D acquisition patterns,” Journal of Nuclear Medicine, vol. 47, pp. 1960-1967, 2006]; and for the animal PET, the spatial resolution is about 1 mm˜2 mm FWHM, the FOV in the vertical axial direction is approximately 10 cm˜15 cm, the width of the scintillation crystal in the tangential direction is generally about 1 mm˜2 mm [Laforest Richard, Longford Desmond, Siegel Stefan, Newport Danny F., Yap Jeffrey, “Performance evaluation of the microPET-Focus-F120,” in IEEE 2004 Nuclear Science Symposium Conference Record, vol. 5, pp. 2965-2969, 2004] [Cristian C Constantinescu, Jogeshwar Mukherjee, “Performance evaluation of an Inveon PET preclinical scanner,” Physics in Medicine and Biology, vol. 54, pp. 2885-2899, 2009]. In order to obtain a PET with a spatial resolution the same as or higher than that of the animal PET while maintaining large FOV, it is required to use a large number of crystals with finely cut, and the number of the crystals increases in a multiple proportional to the square of the ratio of the radius of detection ring between the two PETs. With the increase of the number of the crystals, more and faster photomultiplier tubes and a large number of back-end electronics channels would be needed, leading to a sharp increase in the cost of the entire PET system.